In this study, the performance of two catalysts, ZSM-5 zeolite and sulfated mordenite, for the production of biodiesel from neem oil using an electrochemical method has been investigated. The structure and performance of the catalysts have been characterized by various methods such as FT-IR, XRD, EDX, BET and NH3-TPD, and the quality of the final product has been evaluated based on ASTM standards. The aim of this research is to develop an economical and efficient process for biodiesel production that benefits from the advantages of electrochemical method and heterogeneous catalysts and moves towards renewable energies.
Most of the materials used in the experiment, including oleic acid, palmitic acid, stearic acid, methanol (99%), tetrahydrofuran (THF), sodium chloride, sulfuric acid (98%), potassium hydroxide, normal hexane and solid phenolphthalein were purchased from Merck, Germany. Zeolite ZSM-5 was purchased from Sigma-Aldrich, USA. Mordenite was purchased from Zeolite International Company in Semnan and neem seeds were purchased from Bandar Abbas.
All experiments were performed in a glass jacketed reactor. The electrodes used for this study were two graphite electrodes of the same type and size (1 cm × cm2) and with an internal distance of 1 cm.
In this study, chromatographic analysis (GC) was used to qualitatively and quantitatively measure biodiesel production. The model of the device used was 7890A and the capillary columns used in the Rtx-5 MS and HP-5 device. Also, quantitative FT-IR analysis was used to identify the biodiesel produced, with the FT/IR- Bruker/tensor 27 device model.
Also, in this study, analyses were used to identify the properties of the catalyst. To identify the crystalline phase of the catalyst, functional groups, specific surface area, acidity, morphology and elemental analysis, XRD (with Philips Xpert X-ray diffractometer device model), FT-IR (with FT-IR/ Bruker/tensor 27 device model), BET (with Microtrac BEL Corp device model), NH3-TPD (with Chemisorption Analyzer, NanoSORD (made by Sensiran Co., Iran)), EDX-Mapping (with FESEM device model, ZEISS, Germany, Sigma VP), were used, respectively.
To investigate the extracted fatty acids, GC analysis with mass detector was used. Through GC-MS analysis, it was determined that the most abundant fatty acids in the extracted oil are oleic acid, palmitic acid, and stearic acid. Their respective amounts are reported in Table 1. Eicosanoic acid is another fatty acid present in the extracted oil, which was less than 0.1%.
In this study, the production of biodiesel from FFAs present in synthetic neem oil was investigated and compared by electrochemical method in the presence of two types of zeolite catalysts including Nano-zeolite ZSM-5 and Nano-sulfated mordenite.
The reaction mixture consisted of synthetic neem oil (initial feed), methanol, THF (as co-solvent), sodium chloride (as electrolyte), distilled water and catalyst. All reactions were carried out at ambient temperature. Each of the parameters including molar ratio of methanol to oil at three levels of 10, 20 and 30, voltage applied to the system at levels of 10, 25 and 40 V, reaction time in the range of 1, 2 and 3 hours and catalyst amount of 2, 4 and 6 wt% were tested and finally their optimal amount was obtained and reported. Also, the molar ratio of THF/Oil (cosolvent to oil) was 10:1, the amount of NaCl to oil was 1% by weight, and the amount of distilled water based on the total weight of the solution was 2% by weight. The water in the electrochemical system is hydrolyzed around the anode and cathode electrodes, producing hydrogen ions (reaction 2) and hydroxide ions, respectively. The hydrogen ion produced can act as an acid catalyst for the reaction, the mechanism of which will be examined in the catalysts section. The hydroxide ion produced by the reaction with methanol produces methoxide ion, which is a very strong nucleophile and can act as a base catalyst. Therefore, by hydrolyzing water and simultaneously producing hydroxide and hydrogen ions in the system, both esterification and transesterification processes are carried out simultaneously, so if the feed contains a high amount of acid, it does not cause a problem in the reaction path. Another advantage of electrochemical reactions is that they can be carried out at ambient temperature, saving high energy consumption and being relatively inexpensive [19]. The mechanism of action of both catalysts included the role of Lewis and Brønsted acidic sites in activating the carbonyl group of FFAs and facilitating nucleophilic attack. However, the sulfated mordenite catalyst, due to the presence of sulfate groups with high electronegativity, stronger acidity, and increased surface hydrophobicity, led to more effective adsorption of FFAs and faster removal of produced water molecules, and showed higher efficiency than ZSM-5 zeolite in converting free fatty acids to fatty acid methyl ester (biodiesel). The results of this study show that the choice of zeolite type and its surface modification have a significant impact on the efficiency and speed of the biodiesel production process in electrochemical systems.
 
Conclusion
Biodiesel has the potential to replace fossil fuels. In this study, the esterification process of synthetic neem seed oil was investigated using electrochemical method in the presence of zeolite catalysts. Zeolites have high potential for activity in this type of processes due to the presence of Lewis and Brønsted acidic sites inside and on the surface of their structure.
The findings of this study showed that the sulfuric acid-modified mordenite catalyst (0.5 M sulfated mordenite) performed much better than the ZSM-5 zeolite catalyst in the esterification process of neem seed oil. Analysis of the catalyst characterization results, especially the EDX-Mapping test, revealed the reason for this superiority. The silicon to aluminum (Si/Al) ratio is known as a key indicator in improving the efficiency of zeolite catalysts; increasing this ratio increases the hydrophobicity of the catalyst surface and reduces the possibility of reaction reversal or side reactions. The results showed that the Si/Al ratio for ZSM-5 is 2.22 and for 0.5 M sulfated mordenite is 25.07. In addition, the sulfation process causes the attachment of sulfate functional groups to the catalyst surface. Due to their strong electron-withdrawing properties, these groups significantly strengthen Lewis and Brønsted acids, thereby improving the reaction mechanism.
The effect of operating parameters was investigated using the “one-factor-at-time” method, which showed that the optimal conditions were a voltage of 10 V, a reaction time of 1 h, a molar ratio of methanol to oil of 10:1, and a catalyst amount of 6 wt%. Under these conditions, the biodiesel production efficiency was 85% in the presence of sulfated mordenite and 80% in the presence of ZSM-5. These results indicate that the modification of natural mordenite catalyst by sulfation can be an effective solution for improving the efficiency and performance of biodiesel production processes.